Virus productive replication is dependent on adequate control of pro-apoptotic stimuli triggered by virus infection, gene expression, and replication. Viruses have evolved mechanisms to counter this anti-viral apoptotic response sufficiently to enable efficient virus production. While several anti-apoptotic proteins believed to contribute to this function have been identified in human herpesvirus 8 (HHV-8), these are unlikely to comprise the full repertoire of HHV-8 mechanisms that together effectively counter the cell's anti-viral defenses. Other, non-classical pro-survival viral proteins may include signal-transducing receptors and ligands, such as vGPCR and v-chemokines that we have found promote both cell survival and virus productive replication. In our studies, we have identified Bim, a pro-apoptotic BH3-only Bcl-2 family member, as being induced specifically in lytic reactivation-positive cells, functioning as a key regulator of HHV-8 replication efficiency, and being a target of suppression by the v-chemokines. Furthermore, we have observed that in lytically-infected cells, Bim is localized almost exclusively in the nucleus, indicating an additional and entirely novel mechanism of Bim inhibition, via nuclear sequestration. Such nuclear localization of Bim was seen only in cells supporting HHV-8 lytic reactivation, not in response to other stress-inducers (e.g., serum starvation) that also trigger Bim expression and activation. Investigations to identify viral proteins involved in Bim nuclear translocation have identified viral interferon regulatory factor-1 (vIRF-1) as a central player. This protein, previously reported to form inhibitory interactions with virus-induced cellular interferons and pro-apoptotic p53 and GRIM19, was found to complex with Bim in vitro and in vivo, to promote nuclear translocation and functional inhibition of Bim in transfected cells, and to co-localize with Bim during lytic reactivation in HHV-8 infected endothelial (TIME) cells. This application is focused on characterizing further the physical and functional interactions between Bim and vIRF-1 (Aim 1) and the role of such interactions in HHV-8 productive replication (Aim 2). The work proposed will characterize a novel and important function of vIRF-1 and investigate a new paradigm in the regulation of Bim, a pivotal determinant of HHV-8 replication efficiency. PUBLIC HEALTH RELEVANCE Human herpesvirus 8 (HHV-8) is linked etiologically with the AIDS-associated endothelial tumor Kaposi's sarcoma (KS), very prevalent and aggressive in some countries in Africa, and also with two rare B cell lymphomas, also arising in AIDS patients. Work proposed in this application will explore a novel mechanism of apoptotic inhibition (necessary for efficient virus replication) used by HHV-8, involving a viral interferon regulatory factor (vIRF-1) and its inhibitory interactions with a cellular pro-apoptotic protein, Bim, that is activated in response to cell stress, such as virus infection. Data from this study will provide information of relevance to the development of therapeutic anti-viral strategies.